Review of coastal ecosystem management to improve the health and resilience of the Great Barrier Reef World Heritage Area
Download 0.5 Mb.
|
- Navigate this page:
- Wetlands
- Grasses and sedgelands
- Estuaries
- Coastlines
Impacts to ecosystem functionsForest and woodlandsForests and woodlands on floodplains become periodically inundated during periods of high rainfall and breakout flows from adjoining stream and river channels. Forests and woodlands help to slow water velocity, capture sediment and recycle nutrients while protecting the soil from erosive forces of rainfall and flow.28 Vegetated floodplains are also important areas for regulating groundwater recharge and discharge contributing to the water balance in the landscape. Consequently forested floodplain coastal ecosystems are identified as having a high to medium capacity for a wide range of physical, biogeochemical and biological processes that can deliver ecological functions for the World Heritage Area (Appendix A). The contemporary lower Burdekin floodplain has undergone extensive surface and groundwater hydrological modification. Surface hydrology has been modified by vegetation clearing and land levelling for farm development, the construction of tailwater drains (primarily in the BHWSS), changes to riparian vegetation condition within distributary stream channels, built (road, rail) infrastructure on the floodplain, and by elevated groundwater tables in some areas decreasing the non-saturated zone and reducing infiltration capacity. Cumulative hydrological impacts on the larger floodplain system (including lack of consideration for pre-existing natural drainage depressions networks or preferential flow paths during past land levelling and development of drainage infrastructure), has resulted in reduced interaction of floodplain water run-off and flow with forests or woodlands in a manner that supports the full capacity for vegetated ecosystem functional processes. Flood water flowing from the top of the floodplain to the bottom is described by local stakeholders to now ‘take hours instead of days’. Flows are concentrated by infrastructure or drains, often bypassing and short circuiting remaining drainage depressions, where flows historically slowed and spread across the landscape. Instead of entering distributary streams via broad natural drainage depressions with distributed flow paths, constructed drains deliver concentrated flows often resulting in greater erosion potential. Higher flow velocities and reduced residence time for flood water on forested floodplains limits the capacity for many of the physical and biochemical processes to occur (for example, those associated with flow detention, sediment trapping, and nutrient uptake). Farm development has extended into shallow drainage depressions that previously acted as preferential flow paths during periods of floodplain inundation and has contributed to unco-ordinated drainage patterns, including flood preferential flow paths through developed areas.12 Un-coordinated drainage also affects remnant floodplain forests and woodlands by creating areas artificially watered by irrigation and tailwater. The resulting higher soil moisture and nutrient status can enhance understorey growth including invasive and pyrophytic pasture grasses.20 One other key factor affecting the capacity of remnant forests and woodlands to deliver ecosystem functions and processes (Appendix A) is the shape, location and extent of coastal ecosystem remnants. Narrow linear corridors and small areas have a large external boundary to core area ratio and are more vulnerable to edge effects, including weed invasion and hydrological change associated with neighbouring land uses. Small isolated stands alienated from floodplain overland flow and/or drainage paths have little capacity to deliver flow detention, silt trapping and landscape water balance functions. When you have a situation where the extent of the catchment is extensively cleared and developed dwarfs receiving remnant areas, the capacity of the smaller remnant area to physically or ecologically process received run-off or flows can be overwhelmed (Appendix B). WetlandsFreshwater wetlands are usually associated with areas subject to periodic flooding where standing freshwater persists for at least part of the year, in most years. These areas slow the overland flow of water and cycle nutrients and trap sediment. Freshwater wetlands are also important dry season refugia for a host of aquatic and semi-aquatic species and are used by some marine species with links to the World Heritage Area for parts of their life history.28 Freshwater wetland coastal ecosystems are identified to have one of the highest capacities of any coastal ecosystem types for a wide range of physical, biogeochemical and biological processes that can deliver ecological functions for the World Heritage Area (Appendix A). Palustrine wetlands have experienced the greatest losses to agricultural development ( 6). Historically, this type of wetland system was abundantly distributed along drainage depressions across the lower Burdekin floodplain. With loss of palustrine wetlands, the ability to continue to deliver ecological functions for downstream receiving estuarine and marine systems via processes such as flow detention and sediment retention needs to be considered in terms of the cumulative functional contributions of the chain of palustrine wetlands that used to intercept floodplain run-off. Under perennial and near stable wetland water level conditions created by the hydrological modification of the floodplain, a suite of aquatic weeds create wetland choking weed mats that in the absence of active management can cover the entire water surface of even deep riverine wetlands.29 Formed weed mats shade out and exclude native submerged and bankside emergent macrophytes which have important habitat values. Anoxic water conditions created by the water surface smothering of such weed infestations and the associated organic loading of the water column generate fish kills and create permanent fish passage barriers in floodplain distributary streams. Secondary water quality impacts include the enhanced liberation of phosphorus from bottom sediments under the created anoxic conditions.13 While aquatic weed choked wetlands may have a potentially enhanced capacity for some wetland ecosystem functions (e.g. flow detention, sediment trapping and nutrient uptake) this needs to be weighed against ecological impacts. These include degraded water quality, alienation of upstream fish habitats and impacts associated with the export of anoxic blackwater and delivery of large accumulated organic loads to downstream estuarine and marine systems during peak flows associated with flood events. Bunded coastal wetlands are a particular class of artificial wetland often created in areas that were previously seasonally brackish. Prior to the establishment of perennial irrigation supplies with the Burdekin Falls Dam, most bunded coastal areas still drew down or dried up in the dry season promoting seasonal native macrophyte communities. Seasonal drying facilitating cattle access to weed species, the desiccation and incorporation of organic loads in bed sediments and the creation of open distributary channels prior to wet season flow events. Due to the impacts noted above, effective recruitment of catadromous fish species up distributary streams of the lower Burdekin floodplain has not been demonstrated since the early 1990s30 and recent records for migratory species such a barramundi are attributed to stocking efforts13. The lack of effective fish passage on the lower reaches of the main distributary streams of the lower Burdekin floodplain represent a major condition impact on the migratory fish populations of the areas riverine wetland systems.
Because sedges are aquatic plants they do not decompose and produce high BOD when inundated (in contrast to ponded pasture grasses), and have characteristically high dissolved oxygen concentrations in contained water bodies, also promoted by algal growth on sedge stems. The high productivity and good water quality of inundated sedgelands make them high value nursery habitats for fish including juvenile barramundi which recruit into these areas from adjoining estuarine systems during the wet season and can be found in high abundances in these habitat types. EstuariesEstuaries mix and accumulate organic and mineral nutrients from the land and sea and are consequently highly productive environments and important for recycling nutrients. The fertile and protected environment provided by estuaries provides nurseries for fish and crustacean species that are directly linked to commercial and recreational fisheries is in the Burdekin coastal region.26,31 Many estuarine fish species also have life cycle linkages with Great Barrier Reef habitats. The capacity of estuarine ecosystems to assimilate sediment provides an important water quality role on the margin of the World Heritage Area, as does their ability to biogeochemically modify chemicals and heavy metals. They are also one of the most effective natural carbon sinks.28 Mangrove forests and other estuarine vegetation communities also provide physical protection of the coastline providing security for landward coastal ecosystem and production systems. Besides functional values, estuarine ecosystems of the lower Burdekin floodplain also have high biodiversity and nature conservation values. Burdekin coastal estuaries are recognised for their high mangrove species diversity.26 Conservation values are recognised by inclusion of a significant proportion of lower Burdekin floodplain estuarine coastal ecosystems in protected areas or recognised as areas of specific value:
The lower Burdekin floodplain estuarine wetlands support a number of Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act) listed species including inshore dolphins, saw sharks, saltwater crocodiles and migratory water birds. Shorelines of the lower Burdekin floodplain provide nesting grounds for EPBC Act listed seabirds and marine turtles.32 Shallow aquifers in some beach ridges (i.e. delta coastal margins) also support beach ridge vegetation communities which are EPBC Act listed endangered ecological communities. The high functional, productive and biodiversity values of the lower Burdekin floodplain coastline is reflected in substantial areas of it being protected within National Park, Declared Fish Habitat Areas and Ramsar listing for Internationally Important wetlands (Figure 11). While much of the lower Burdekin floodplain coastline is relatively isolated from areas of human settlement it is still vulnerable to a number of local, regional and more global drivers of condition impact. Even remote coastline ecosystems of the lower Burdekin floodplain are subject to impact risks to biota posed by marine debris. High levels of recreational boat and vehicle ownership also mean that local residents and visitors to the region alike have relatively unregulated access to much of the beach coastline posing risks to matters of national environmental significance. Damage to dune vegetation by quad bikes and associated erosion risks and disturbances to nesting birds and turtles has been noted at Alva Beach.26 Impacts to coastal processes including the supply of beach forming sand attributed to coastal development and dam infrastructure in the Burdekin basin have been cited in relation to changes in coastal landforms adjoining the Kalamia Creek mouth.26 Like many coastal ecosystems downstream of intensive agriculture in the lower Burdekin floodplain, shoreline sediment samples recorded at the mouth of the Burdekin River have also contained agri-chemical residues associated with dichloro-diphenyl-trichloroethane (DDT).33 Some of the most significant changes to the lower Burdekin floodplain coastline are associated with cyclones and storm surge events. Both southern Bowling Green Bay and Cape Bowling Green Bay have been identified as being particularly vulnerable to erosion. The latter is known to host large populations of EPBC listed migratory wader birds.32 Predicted increases in the severity of cyclonic events in tropical north Queensland associated with the impacts of global warming highlights the importance of ensuring all available management avenues are pursued to increase the resilience of these coastline ecosystems.
Directory: jspui -> bitstream -> 11017 bitstream -> A. gw student and alumni numbers summary 3 bitstream -> The deepest gratitude to my supervisor Professor George Panayiotakis for offering me the opportunity to make this PhD and for his continuous support and guidance during all these years bitstream -> Advances in Imaging from the first x-ray Images bitstream -> Clustering Microarray Data within Amorphous Computing Paradigm and Growing Neural Gas Algorithm bitstream -> Навчальний посібник Для студентів економічних І правових спеціальностей немовних вузів Суми двнз "уабс нбу" 2014 Download 0.5 Mb. Share with your friends:
|
